1. Field of the Invention
This invention relates to a high efficiency radio-broadcasting transmitter optimized for digital type transmissions. It is applicable particularly to short wave radio broadcasting transmissions.
2. Description of Related Art
Radio broadcasting transmitters used at the present time for short wave transmissions are optimized to have a very high efficiency during transmission in pure amplitude modulation with carrier residue.
In order to achieve this, they are organized around a high power tube acting essentially as a current switcher at the rate of a carrier wave to be transmitted. A high voltage signal proportional to the instantaneous amplitude of the high frequency wave to be transmitted is applied to the anode tube through the output from a modulator. Modulations used at the present time are known as abbreviations IML and PSM.
With new digital radio broadcasting systems currently being standardized, the transmitted wave shape is not related to the audio frequency signal to be transmitted. It is of the type that is used in serial or parallel modulators. The binary stream that is transported depends on the coding of the audio frequency signal that is done on the input side, and the data that accompany it. The purpose of the process is to significantly improve the intrinsic reception quality of audio frequency signals and to make them insensitive to unwanted effects that occurred during propagation and are due mainly to fading, noise and interference phenomena, provided that they remain limited to reasonable values.
Another advantage of the process is that there is no need to transmit a carrier wave, although the carrier wave represents up to 90% of the total transmitted power in amplitude modulation transmitters. Furthermore, with a digital modulation process with a serial or parallel modulator, the transmitted signal is modulated both in amplitude and in phase. It is a complex signal, usually described by the relation S(t)=1(t)+j Q(t), where 1(t) is the phase signal and Q(t) is the quadrature signal. This makes it possible to consider using a conventional amplitude modulation transmitter in which a frequency reference is modulated in phase and in which the input audio frequency signal is proportional to the modulus of the complex signal to be transmitted.
Tests carried out up to now on this type of transmitter show that although the transmitted signal quality can be considered as being sufficient for reception, it is insufficient for an operational system that will need to cohabit with other transmitters, regardless of whether they are amplitude or digital modulation signals.
Even if the precaution of transmitting a carrier residue is taken in order to make operation of transmitter performances linear in terms of distortion, pass band and neutralization, the result is that the parasite transmissions in channels adjacent to the channels used by the transmitter are too high.
Defects with this type of transmitter are essentially due to the fact that the signal to be transmitted has all the characteristics of Gaussian or quasi Gaussian noise at its usual location, in other words close to the origin for I=0 and Q=0, whereas this is precisely the location at which difficulties are greatest.
The phase varies fastest when the signal passes close to the origin, which automatically makes it necessary to have a wide passband for the phase modulated channel.
Furthermore, there are also sharp direction changes in the amplitude channel close to the origin, which also need a wide passband of the amplitude channel, typically equal to at least three times the bandwidth of the transmitted signal.
However, the main concern of transmitter manufacturers is efficiency, which is usually given priority over linearity and phase distortion due to the approximate neutralization of the output tube.
Therefore the problem with the transmission of a digital type signal cannot be solved by simply adding control signals on existing transmitters. It requires the design of appropriate transmitters, however their efficiency must be acceptable for the operator and they must also be able to continue to transmit in pure amplitude modulation if necessary, in at least one transient phase.
For example, it would be possible to consider solving this problem when using a class A transmitter, in other words for which the transmission tube operates under unsaturated conditions, or to use the solution of transmitters known as DOHERTY transmitters.
A class A transmitter may be considered as being a pure amplifier in which the input is modulated by a low level high frequency signal and that outputs a high level replica of the input signal on its output, that is directly injected into the transmitter antenna system.
Unfortunately, apart from its deplorable efficiency of between 20 and 25% maximum, this system cannot be used for the main reason that there is no power tube within the useable 100 kW class, since the tubes are optimized to operate in class C in which energy efficiency is optimal.
DOHERTY type transmitters use two coupled tubes operating in a high efficiency mode. For illustration, a 90 kW transmitter of this type marketed by RCA is still in operation in the Vatican radio broadcasting station. This transmitter comprises two symmetric phase modulated tubes, and an output formed by a combination of the outputs from the two tubes which is exclusively amplitude modulated, however with a residual phase modulation that is not perceptible to existing receivers on the market. In this case too, this assembly is dedicated to radio broadcasting in amplitude modulation with carrier residue. But economically it is not very attractive, since it requires the use of two transmitter output power tubes.